The invention concerns a process for producing a coating on a refractory structural member, in which a noble metal alloy is applied as a coating material to the refractory structural member. The noble metal of the noble metal alloy has a melting point greater than 1,400xc2x0 C. and consists of platinum and/or iridium and/or rhodium and/or ruthenium and/or gold. The noble metal alloy is used in the form of a powder, and the refractory structural member is coated with the powder. The invention also relates the use of a coating produced in this way on refractory structural members in the glass industry.
Structural members that are used in the production of glass, especially in the area of glass melting furnaces and feeders, are exposed to high temperatures and corrosive atmospheres. Especially severe corrosive attack occurs, for example, at the three-phase boundary between the molten glass, the gas atmosphere above the molten glass, and the refractory material. This causes the refractory material to erode. Refractory material that has worn away contaminates the molten glass and reduces its quality. To guarantee high quality of the glass and to prolong the useful life of the structural members, it is customary to coat, clad, or line the structural members with a noble metal.
EP 0,559,330 A1 describes such a lining for use in glass tank furnaces and for other purposes. In this case, a nonporous coating made of a noble metal or a noble metal alloy protects a ceramic substrate from corrosive attack by the molten glass and the corrosive atmosphere above the molten glass. The nonporous coating is applied to the substrate by the Schoop process and then consolidated by a mechanical or thermal treatment. The coefficients of thermal expansion of the substrate and the nonporous coating are matched to each other to prevent the coating from becoming detached from the substrate.
EP 0,471,505 B1 and EP 0,679,733 A2 describe structural members that consist of metallic substrates and a coating composed of several metallic and ceramic layers. The last layer of the coating is composed of a noble metal or a noble metal alloy and is nonporous. This last layer is preferably applied by thermal spraying, electrodeposition or in the form of a powder and then consolidated by mechanical and/or heat treatment.
In the coating processes described above, it is necessary to include a process stop in which the last layer, which contains the noble metal and which will come into contact with the molten glass and corrosive atmosphere, is subsequently consolidated and its open porosity is closed. This is disadvantageous and cost-intensive.
DE 196-51,851 C1 describes a process for producing oxide ceramic structural members for the glass industry that are coated with noble metal, preferably platinum. A dense and adherent layer is produced here by using a baking paste, which contains platinum particles with an average initial particle size of xe2x89xa610 xcexcm. The particles are converted to platelets by cold deformation with a degree of deformation of "PHgr"xe2x89xa72.5 and baked in an oxidizing atmosphere by a continuous temperature-time program. Layer thicknesses of up to about 100 xcexcm are achieved in a cycle. Although a subsequent consolidation of the baked layer is not necessary here, the cold deformation of the platinum particles is also an expensive process step.
Therefore, the object of the present invention is to provide a faster and more cost-effective process for producing coatings on structural members for the glass industry and to specify a use for a coating of this type.
This object is achieved with a process in which the noble metal alloy has a liquidus temperature TL of 900xc2x0 C. to 1,400xc2x0 C., and the noble metal alloy contains a noble metal in an amount of xe2x89xa784 to xe2x89xa699.5 wt. % and an oxidizable substance in an amount of xe2x89xa70.5 to xe2x89xa616 wt. %. The oxidizable substance includes boron and/or phosphorus and/or antimony and/or arsenic
The refractory structural member and the coating are heated at least once in an oxygen-containing atmosphere to a temperature T that is greater than or equal to the liquidus temperature TL of the noble metal alloy. The oxidizable substance is oxidized during this heating process, and the oxide that has formed is at least partially vaporized. The temperature T is maintained until the proportion of oxidizable substance in the coating is  less than 0.1 atomic percent, and the coated refractory structural member is then cooled.
Accordingly, in the inventive process, a noble metal alloy that consists of a high-melting noble metal or metals and an oxidizable substance that acts as a flux is melted, which requires a temperature below the melting point of the noble metal(s) that are used.
Noble metal alloys with boron have already been described, e.g., in DE OS 1,558,902 for producing solder joints between carbon materials and other materials, or in U.S. Pat. No. 7,087,932 for coating graphite. Noble metal alloys with phosphorus for the joining of structural members have also been described in JP 63[1988]-139,072. In addition, EP 0,209,264 describes an amorphous rhodium alloy with boron, phosphorus, or arsenic.
When the noble metal alloy melts, the oxidizable substances cause complete wetting of the refractory structural members and result in the formation of a dense coating that strongly adheres to the structural member.
However, in the process of the invention, the oxidizable substances in the molten noble metal alloy react with the oxygen in the oxygen-containing atmosphere to form an oxide, which vaporizes. This vaporization causes the content of oxidizable substance in the coating to decrease. The time allowed for this vaporization process to proceed can be selected so that the oxidizable substance is virtually completely removed. In the present case, the time is selected so that the coating of noble metal that is left behind contains  less than 0.1 atomic percent of the oxidizable substance. The melting point of the finished coating is thus almost the same as the melting point of the noble metal used in the noble metal alloy. Accordingly, the process of the invention has the advantage that the coating can be produced at relatively low temperatures but used at much higher temperatures.
It is especially preferred for the noble metal to include xe2x89xa770 wt. % platinum and xe2x89xa630 wt. % gold and/or iridium and/or rhodium.
The refractory structural member may be a ceramic or a metal. Preferred ceramic materials are Al2O3 and/or SiO2 and/or ZrO2 and/or zirconium silicate and/or aluminum silicate.
Molybdenum and/or iron and/or nickel and/or cobalt may be used as the metal. The resistance of these metals to oxidation can be enhanced if the metal is composed of iron and/or nickel and/or cobalt and 15 to 30 wt. % of aluminum and/or chromium. The scaling tendency of the metal can also be reduced by adding 0.01 to 0.3 wt. % of hafnium and/or yttrium and/or lanthanum and/or cerium, or one or more of their oxides (hafnium oxide, yttrium oxide, lanthanum oxide, cerium oxide). In addition, the metal may contain niobium, titanium, or silicon. The metal may also be coated with a ceramic coating before it is coated with the noble metal alloy. Here again, preferred ceramic materials for this purpose are Al2O3 and/or SiO2 and/or ZrO2 and/or zirconium silicate and/or aluminum silicate. This ceramic coating may be applied by vapor deposition, sputtering, or plasma spraying.
Especially strongly bonded coatings are produced when the maximum particle size of the noble metal alloy powder is 150 xcexcm. Ideally, the maximum particle size is 50 xcexcm.
Selection of the liquidus temperature TL in the range of 1,100xc2x0 C. to 1,300xc2x0 C. was found to be effective. The oxide of the oxidizable substance vaporizes quickly in this temperature range, and the required temperatures can be readily achieved in standard furnaces.
To produce the coating, the refractory structural member can be coated with the powder by spraying the powder onto it or by applying the powder in the form of a suspension. In the latter case, the refractory structural member can be coated by dipping it into the suspension or by spraying, brushing, or pressing the suspension onto the refractory structural member. These processes are more cost-effective than the previously used process of thermal spraying.
The use of a noble metal alloy composed of platinum as the noble metal and boron as the oxidizable substance was found to be especially effective in the process of the invention. It is advantageous for boron to be present in the noble metal alloy in amounts of 0.5 to 2 wt. %.
The use of a noble metal alloy composed of platinum as the noble metal and phosphorus as the oxidizable substance was also found to be effective. In this case, the noble metal alloy preferably contains 2 to 3.5 wt. % of phosphorus.
However, it is also possible to use a noble metal alloy composed of platinum as the noble metal and antimony as the oxidizable substance. Antimony is preferably contained in the noble metal alloy in amounts of about 8 to about 16 wt. %. It is also possible to use a noble metal alloy composed of platinum as the noble metal and arsenic as the oxidizable substance. This noble metal alloy should contain about 5 to about 10 wt. % of arsenic.
Of course, since antimony and arsenic are highly toxic substances, the use of a noble metal alloy with boron and/or phosphorus is preferred.
The coating can be made especially dense by heating the refractory structural member and the coating once to a temperature T1 that is greater than or equal to the liquidus temperature TL of the noble metal alloy and then heating the refractory member and the coating a second time to a temperature T2 that is greater than T1.
Temperature T1 is preferably selected below 1,400xc2x0 C., and temperature T2 is preferably selected above 1,400xc2x0 C. Temperature T2 is preferably maintained until the proportion of oxidizable substance in the coating is  less than 0.01 atomic percent and ideally  less than 0.005 atomic percent.
The refractory structural member is preferably coated with the powdered noble metal alloy in a thickness such that, after heating, the coating has a thickness of 50 to 500 xcexcm.
The refractory structural member may be completely or only partially covered with the coating. For example, a structural member may be coated only in the region of the three-phase boundary between the molten glass, the gas atmosphere above the molten glass, and the refractory material, which is the region that is exposed to strong corrosive attack.
The oxygen-containing atmosphere is preferably air or oxygen. Naturally, however, it is also possible to use other gas mixtures that contain oxygen, such as mixtures of argon and oxygen or mixtures of nitrogen and oxygen.
After it has cooled, the coating may also be further coated by electrodeposition to increase the layer thickness or to modify the properties.
The goal of the invention with respect to the use of the coating of the invention is achieved by specifying that a coating produced on a refractory structural member by the process of the invention is used in contact with molten glass.